/Engine/Source/Runtime/SlateCore/Public/Input/NavigationReply.h
An event should return aFNavigationReply::CustomBoundary()to let the system know to call a custom delegate to get the widget to navigate to is the widgets boundary is hit.
static [FNavigationReply](API\Runtime\SlateCore\Input\FNavigationReply) CustomBoundary ( const FNavigationDelegate & InFocusDelegate )